Robust dual membrane microphone

ABSTRACT

A dual membrane microphone is disclosed. In an embodiment, a MEMS microphone includes a first membrane, a backplate with a separated central area including a first backplate electrode on a lower portion of the backplate, a second backplate electrode on a upper portion of the backplate and a backplate insulation layer galvanically isolating the first and the second backplate electrodes, a second membrane and a coupling central portion, wherein the first membrane couples mechanically to the separated central area of the backplate in an electrically isolating manner and the separated central area of the backplate couples to the second membrane in an electrically isolating manner.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to German Patent Application No.102019123077.2, filed on Aug. 28, 2019, which application is herebyincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to improved MEMS microphones and methodsfor the production of such microphones.

BACKGROUND

MEMS microphones typically have a capacitor formed by a fixed and rigidelectrode embodied in a planar fashion, the so-called backplate, and afurther electrode embodied in a planar fashion and arranged in parallelthat is flexible and able to oscillate forming the membrane ordiaphragm.

If acoustic pressure waves impinge the microphone the flexible membranemay oscillate relative to the backplate changing the capacitance overtime. A bias voltage (Vb) applied to the backplate (or the membrane)facilitates measuring sound pressure induced deflections of the membraneas a capacitance change results in a voltage change and thereby isproviding a useful signal for further processing.

As the quality of a signal output can be greatly improved byimplementing differential signal readout, in more sophisticated MEMSmicrophones often an arrangement with two capacitive elements is used.Subtracting or adding the signals of both capacitive elements canenhance the signal-to-noise ratio (SNR) and reduce the total harmonicdistortion (THD) even at high sound pressure (130 dB).

In U.S. Pat. No. 8,664,733 B2 a MEMS microphone with a dual backplateconstruction is disclosed. A flexible membrane is positioned in betweentwo outer backplate electrodes. The differential signal readout of thetwo capacitances improves the sensitivity of the microphone.

As the outer backplates inhibit the sensitivity, they often have largeholes (5-20 μm) to decrease the acoustic resistance. Therefore, duringproduction, packaging or usage particles might enter the air gap betweenmembrane and backplate through the holes and restrict the movement ofthe membrane relative to the backplate.

U.S. Patent Application No. 2017/0230757 A1 discloses a MEMS microphonewith a backplate positioned between two detached membranes allowing togenerate a differential signal to enhance the sensitivity of themicrophone. Due to a floating membrane construction the MEMS microphonehas open gaps at the edge of the membranes allowing particles to getstuck or to enter the airgap. A particle in the airgap can hinder thedisplacement of the membrane, leading to a lower acoustic signal andcausing a bad THD performance. Additionally, as the backplate has justone electrode for both capacitances, formed with the two membranes asthe other electrodes, the bias voltages cannot be tuned independently.

U.S. Pat. No. 9,828,237 B2 discloses a MEMS device with a backplatepossessing two adjacent electrode portions electrically isolated fromeach other and placed in between two coupled membranes while a lowpressure region is formed in between the membranes. As the two membranesare hermetically sealing the airgap to sustain the low pressure regionmany pillars are necessary to couple the membranes with each other tomaintain a constant distance of the membranes to the backplate, even atstatic pressure changes. The noise level in such an acoustically stiffsystem is high due to the low signal amplitude and the high correlationcoefficient of the membranes, which annihilates the advantages of thedifferential signal readout.

SUMMARY

Embodiments provide a MEMS microphone with a differential signal readoutwhich eliminates the drawbacks, e.g., particles in the airgap or anacoustic stiff system, as mentioned above.

Embodiments provide a MEMS microphone with two membranes that are spacedapart but are mutually coupled mechanically just in the center area ofthe membranes. In between these membranes a backplate comprising twoelectrically isolated electrodes on an upper and a lower surfacerespectively is positioned. When an acoustic pressure wave impinges themembranes a differential signal can be readout at four output ports thatare coupled to the two membranes and the two backplate electrodesrespectively.

As the membranes are mechanically coupled only in the center area of themembrane the construction does have, compared to known architectures ofMEMS microphones with two membranes requiring a low pressure and hencemany pillars in between the membranes, a smaller correlation coefficientfor the two membranes. Therefore the noise level can be reduced and theSNR enhanced.

While a dual-backplate construction with a flexible membrane in betweentwo backplates requires large holes in the backplates to reduce theacoustic resistance, embodiments with the dual-membrane construction canhave just one hole, with a diameter smaller than 0.5 μm, in the firstmembrane facing the sound port, the second membrane, or both membranes.Alternatively, a multitude of holes smaller than 0.5 μm can be arrangedone the first and second membrane. Thus, after production, as the holesare smaller than the typical particle size, no particles can enter theairgap between the membrane and the backplate. Consequentlydeterioration of sensitivity and THD performance due to particles in theairgap can be avoided and a higher durability and a longer lifetimeachieved.

Further embodiments provide two electrically separated backplateelectrodes each facing towards a respective one of the membranes. Thisallows adjusting the bias voltage for both capacitive elementscompletely independently. Hence, production tolerances regarding theairgap or tension variations in the membranes can be compensated byadapting the two bias voltages resulting in a better THD performance.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention and a method of manufacture is describedbased on embodiments with reference to the figures. Same parts or partswith equivalent effect are referred to by the same reference numbers.

The figures serve solely to illustrate the invention and are thereforeonly schematic and not drawn to scale. Some parts may be exaggerated ordistorted in the dimensions. Therefore, neither absolute nor relativedimensions can be taken from the figures. Identical or identicallyacting parts are provided with the same reference numerals.

FIG. 1 shows a cross section through a substrate coated with a firstpartial layer construction;

FIG. 2 shows a cross section through said construction with a patternedfirst functional layer thereon;

FIG. 3 shows the arrangement after edge-covering deposition of aninsulation layer;

FIG. 4 shows the arrangement after planarization;

FIG. 5 shows the arrangement after the production of a second functionallayer;

FIG. 6 shows the arrangement after patterning the second functionallayer;

FIG. 7 shows the arrangement after the production of the thirdfunctional layer;

FIG. 8 shows the arrangement after patterning of the third functionallayer;

FIG. 9 shows the arrangement after the production of a furtherinsulation layer and the opening of the contact holes;

FIG. 10 shows the arrangement after production of contact holes andcontacts therein;

FIG. 11 shows the arrangement after the production of a perforationthrough the silicon substrate;

FIG. 12 shows the finished microphone after the removing the sacrificiallayers in the active region;

FIG. 13 shows a finished microphone in accordance with a secondembodiment;

FIG. 14 shows a finished microphone in accordance with a thirdembodiment;

FIG. 15 shows a finished microphone in accordance with a fifthembodiment; and

FIGS. 16a-e show a schematic planar cross section of a) a firstmembrane; b) a coupling element between a first membrane and thebackplate; c) the backplate; d) a coupling element between the backplateand a second membrane; e) a second membrane of the second embodimentshown in FIG. 13.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

One advantage of the described MEMS structure/MEMS microphone is thatthe production thereof can be done a simple method. Contrary to commonlyused production methods, the new method differs in applying all partiallayers of the layer construction sequentially one above another andpatterning them from the top, i.e., that means only from one side. Justthe substrate is etched from the other side after the layer constructionhas been produced thereupon to provide a freely oscillating region abovethe perforation in the substrate.

Sacrificial layers, preferably made out of silicon oxide, applied in theoriginal layer construction between the functional layers, are etchedfrom the top and bottom by wet etching at the same time. At least in thefreely oscillating region the sacrificial layers are removed between thefunctional layers to excavate a free space creating the requireddistance between the functional layers.

The method comprises following steps:

a) On a substrate a first insulation layer IL1 and one or a plurality ofpartial layers for the first membrane M1 are deposited.

b) The partial layers of the first membrane M1 are patterned bylithographic etching.

c) Above the first membrane M1 a second insulation layer IL2 and aplurality of partial layers for the backplate BP are deposited while thetwo conductive partial layers as backplate electrodes (BPE1-2) aregalvanically isolated from each other.

d) The partial layers of the backplate BP are patterned by lithographicetching, separating a central area SCA in the center of the backplateBP, which the horizontal radial extension of which is large compared tothe vertical thickness of the second insulating layer IL2.

e) A third insulation layer IL3 and one or a plurality of partial layersfor a second membrane M2 are deposited.

f) The second membrane M2 is patterned by lithographic etching.

g) A fourth insulation layer IL4 is deposited above the second membraneM2.

h) Openings OP1-4 to the electrically conductive partial layers of thefirst membrane M1, the two backplate electrodes BPE1-2 and theconductive partial layer of the second membrane M2 are etched andelectrical contact pads CO1-4 are deposited therein.

i) A perforation PE is etched through the substrate SU from the bottomside of the substrate SU below an active region of the membranes toexpose the layer stack formed in step a) below an active region of themembranes.

j) The insulation layers are removed in the active region by sacrificialetching (wet- or vapor-etch) creating an airgap between both membranesM1-2 and the backplate BP.

k) Sacrificial etching (wet- or vapor-etch) is stopped, sparing aportion of the insulation layers under and above the separated centralarea SCA in the center of the backplate BP etched in step d) to createtwo electrically isolating volumes coupling mechanically the separatedcentral area SCA to both membranes M1-2 forming a rigid mechanicaljoint, the central coupling portion CCP.

All partial layers of the layer construction are first deposited toextend over the complete area of a respective underlying surface, whichmay be planarized. Wet etching or reactive ion etching (RIE) is thenused for patterning the functional layers providing a good selectivityrelative to the insulation layers underneath. Other anisotropicallyworking etching methods are possible too.

The first insulation layer IL1 serves as an etch stop layer for theetching of the perforation into the silicon substrate from the surfaceopposing the layer stack. The insulation layers deposited between thefunctional layers serve as sacrificial layers that are removed later bywet etching in the active region above the perforation in the siliconsubstrate during method step j).

Protection of the second membrane M2 during etching of the openings tothe conducting partial layers, the production of the contact pads instep h), etching of perforation in the silicon substrate in step j), aswell as the removal of the sacrificial layers between the functionallayers in step g) is provided by the fourth insulation layer IL4. Apreferred material for all insulation and sacrificial layers is siliconoxide, which can be applied in an LPCVD method (low pressure chemicalvapor deposition). In principle, however, other insulation layers andsacrificial layers are also suitable, particularly if they can be etchedselectively.

Doped polysilicon is a suitable material to manufacture the electricallyconductive partial layers of the functional layers. Therefore thepolysilicon has to be doped which can be done with a p-type or n-typedopant, for example with boron or phosphorus. Nonetheless, other dopantsare applicable for doping the polysilicon layer too as far as they canprovide sufficient conductivity.

Silicon nitride layers are used as a mechanical structure and aselectrical insulation in other partial layers of the functional layers.Both membranes preferably have a symmetrical structure which facilitatea linear deflection in both directions and increases the mechanicalstability of the membranes. In a preferred embodiment both membranestherefore comprise three partial layers, each arranged in such a waythat the mentioned above highly doped polysilicon layer has a siliconnitride layer adjacent to it on both sides respectively.

The backplate BP, located in between both membranes M1-2, on the otherhand requires two backplate electrodes BPE1-2. In a simple embodimentthe backplate BP consists of an insulating silicon nitride layer withtwo highly doped polysilicon layer applied on opposing surfaces thereof,each facing a corresponding membrane.

FIG. 1 shows in schematic cross section a substrate SU with a firstpartial layer construction comprising a first insulation layer IL1 andthe first functional layer, the first membrane M1. Silicon providingsufficient stability in the case of a thickness of approximately 200-750μm, serves as substrate, for instance. Thereabove, in a first LPCVDmethod, a first insulation layer IL1 is applied, for example a 1 μmthick SiO₂ layer deposited in a TEOS method. The first functional layerthat is the first the membrane M1 is shown here as one layer, for thesake of simplicity, but may consist in a preferred embodiment of e.g.three partial layers, namely a highly doped polysilicon layer in betweentwo silicon nitride layers. As a first and a third partial layer of themembrane M1, one or a plurality of silicon nitride layers is appliedwith a total layer thickness of about 0.1 μm, for example, in an LPCVDmethod.

The process is controlled such that the silicon nitride layer has asuperstoichiometric content of silicon. One or a plurality of highlydoped polysilicon layers, likewise applied using LPCVD deposition,serves as second partial layer of the first membrane M1. Duringapplication, the polysilicon layer is highly doped in situ with an n- ora p-type dopant (e.g. boron or phosphorus), that is to say provided witha B++ or P++ doping. FIG. 1 shows the arrangement at this method stage.The cross-sectional symmetrical construction of the membrane preventsthe partial layers from being strained asymmetrically and the membranefrom warping after being etched through on account of the strain.

FIG. 2 shows the arrangement after the patterning of the first membraneM1. For the purpose of patterning, a photoresist is applied anddeveloped, thereby exposing undesired regions of the layers formingmembrane M1 which are then removed, respectively. In particular, themembrane is a really delimited and provided with optional small ventingholes VH1, smaller than 0.5 μm in diameter, in the freely oscillatingregion. Said holes enable access for the etchant during later removal ofthe sacrificial layer. Due to the small size of the venting holes VH1 noparticles larger than 0.5 μm can enter the microphone after production.

Noteworthy is the large non-patterned region in the central area CA ofthe membrane M1, which later becomes a part of the coupling centralportion CPP during the manufacturing. The patterning of the membrane M1is accomplished by means of an RIE etching process, for example.

FIG. 3 shows the arrangement after the deposition of a furtherinsulation layer IL. This is done once again in a TEOS LPCVD process.The layer thickness of said further insulation layer is dimensioned suchthat, firstly, the venting holes VH1 are completely overgrown withsilicon oxide and, secondly, the total height of the insulation layerreaches at least the level of the top side of the membrane M1. Theinsulation layer IL and first insulation layer IL1 combine on account ofidentical material and deposition conditions to form a homogenous layer,which is depicted in the figure by virtue of the fact that nodemarcation between first and further insulation layers is illustrated.

FIG. 4 shows the arrangement after a planarization process, in which theinsulation layer is ground from the top down to the level of the upperedge of the membrane M1. By way of example, a CMP method can be used forthis purpose.

FIG. 5 shows the arrangement after the deposition of a second insulationlayer IL2 and three partial layers for the backplate BP. The secondinsulation layer is again applied as a SiO₂ layer in a TEOS LPCVDmethod. For the backplate BP, firstly as a first partial layer the firstbackplate electrode BPE1 is applied as a highly doped polysilicon layerin an LPCVD method. Thereabove, as second partial layer the backplateisolation BPI, a non-conductive silicon nitride layer is applied, whichcan once again be deposited in an LPCVD method. The topmost and thirdpartial layer, the second backplate electrode BPE2 of the backplate BPis once again a highly doped polysilicon layer, which is applied in aknown manner. The polysilicon layers can be a separated structure todefine the electrode areas.

FIG. 6 shows a first cross section through the arrangement after thepatterning of the backplate BP. For this purpose, a lithography step iscarried out and the patterning is carried out in an RIE etching methoddesigned for etching polysilicon and silicon nitride. If appropriate,the layers can be etched under different conditions by continuouslychanging the etching atmosphere. The backplate holes VH2 have a diameterof about 5-20 micrometer and are large compared to the small ventingholes VH1 in the membrane, which are smaller than 0.5 μm, to reduce theacoustic resistance within the device. In the central area of thebackplate BP an incision through all of the three partial layerssurrounds and encloses a separated coupling area SCA, which is notattached to the rest of the backplate BP anymore. The separated couplingarea SCA becomes a portion of the coupling between the membranes lateron.

FIG. 7 shows the arrangement after the deposition of at least onefurther insulation layer and the second membrane M2. After planarizationof this insulation layer, as described before for FIG. 4, a thirdinsulation layer IL3 in the form of a SiO₂ layer is applied to thebackplate BP in a TEOS LPCVD method. For the second membrane M2, whichis shown in FIG. 7 for the sake of simplicity as one single layer,another three partial layers, namely a highly doped polysilicon layer inbetween two silicon nitride layers are applied to the third insulationlayer IL3 using a conformal LPCVD method.

FIG. 8 shows the arrangement after patterning of the layers forming thesecond membrane M2, respectively, which can once again be carried out ina manner supported lithographically by means of an RIE etching method.In this case, the third insulation layer IL3 serves as an etching stoplayer. In the freely oscillating region, the second membrane M2 likewiseprovided with small venting holes VH1 that fulfill the same purpose,already mentioned, as the venting holes VH1 in the first membrane M1. Inthe next step, a fourth insulation layer IL4 is applied above thepatterned second membrane M2 with a TEOS LPCVD method. This layercompletely covers the patterned second membrane M2 including the holesand the patterned edges.

Afterward, various openings OP1-4 are etched into the insulation layers,at the bottom of which holes the conductive partial layers of firstmembrane M1, the second membrane M2 as well as both backplate electrodesBPE1-2 are respectively exposed. The removal of the SiO₂ layers in theopenings is carried out by means of a wet etching step adjustedselectively to SiO₂. In this case, the polysilicon layer of the secondmembrane M2, both backplate electrodes BPE1-2, consisting of polysiliconand the polysilicon layer of the first membrane M1 function as anetching stop layer. In order to uncover the polysilicon layer of themembranes and the first backplate electrode BPE1, the silicon nitridelayer also has to be removed in the corresponding openings, which iscarried out by means of an RIE etching method.

FIG. 9 shows a first opening OP1, in which the polysilicon layer of thefirst membrane M1 is uncovered, a second opening OP2, in which thepolysilicon layer of the second membrane M2 is uncovered, a thirdopening OP3, in which the polysilicon layer of the second backplateelectrode BPE2 is uncovered and a forth opening OP4 uncovering the firstbackplate electrode BPE2. The illustration does not show possiblefurther contact holes to one of the stated functional layers or to thesubstrate SU.

After the next step, shown in FIG. 10, contact pads CO1-4 are producedin the opening OP1-4. These are produced in a method, wherein a basemetallization, a conductive layer and a covering layer are produced oneabove another. The base layer is deposited over the whole area andpatterned. The conductive layer and the covering layer can be grownselectively above the patterned base metallization. Suitable layers are,for example, 1 μm aluminum for the base metallization, 3 μm nickel forthe conductive layer and 300 nm Au for the covering layer. Likewisesolely an Al layer can be used for forming contact pads CO1-4.

FIG. 11 shows a schematic cross section of the layer stack after aperforation PE has been etched anisotropically through the substrate SUby means of a DRIE method. Despite the area provided for the perforation(PE) being on the underside of the substrate SU, all other surfaces arecovered with a protective layer, in particular with the resist used forthe lithography. In the DRIE method, the first insulation layer IL1directly on the substrate serves as an etching stop layer.

The next step then involves removing those regions of the insulationlayers which serve as sacrificial layers, in particular in the freelyoscillating region between the first membrane M1, the backplate BP andthe second membrane M2, and also the remaining insulation layer IL4applied superficially. An isotropic etching process is used to removemost of the sacrificial layers between both membranes M1-2 and thebackplate BP. Well suited for example, a VHF (HF vapor etch) process,which operates with vaporous or gaseous hydrogen fluoride orhydrofluoric acid. This etching process is not quite selective againstpolysilicon and silicon nitride, so that respective partial layers ofthe functional layers are only slightly affected. The removal of thesacrificial layers is done only in the active region, so that themembranes are fixed at the edge by the whole layer composite.

Due to the geometry of the layer construction, in particular because thehorizontal extension of the separated coupling area SCA of the backplateBP is large compared to the vertical distance between the backplate BPand one of the membranes M1-2, the sacrificial layer is not removedcompletely between the membranes M1-2 in the separated coupling areaSCA, but just only at the edges of the separated coupling area SCA. Theremaining portions of the sacrificial layers in between separatedcoupling area SCA the backplate BP and the membranes M1-2 serve as acoupling element between the membranes M1-2 and the separated couplingarea SCA and form together central coupling portion CCP coupling bothmembranes to each other.

In a final step, reactive surfaces can be passivated and are saturatedwith unreactive groups. For doing this, a so-called SAM layer (=selfassembling monolayer), for example can be applied. In this casemolecules containing elongated radicals are bonded to the reactivesurfaces of the microphone by means of a reactive group and form amonolayer, that is, a monomolecular film of the thickness of a molecularlength. The other end of the elongated radical is chemically inert andpreferably also has little physical interaction with other materials. Ifthe inert residue is, for example, a fluorinated alkyl group, then theradicals set up in the monolayer with the inert ends parallel to eachother and extend normal to the surface. Thereby the surface ispassivated, cannot oxidize, corrode, is water and dust resistant, andthereby prevents the membrane M1 to adhere to the backplate BP.

FIG. 12 shows a first cross section through the structure obtained afterpassivation, in which stage the microphone has largely been completed.

FIG. 13 shows a second embodiment of the MEMS microphone.

In this embodiment most of the vent holes VH1 in the first membrane M1are omitted. Instead, one central vent hole VH1 is produced in the firstmembrane M1, in the separated central area SCA and an additional venthole in the center of the second membrane M2. By omitting most of thevent holes the microphone is even more robust as no particles can enterthe airgaps between the membrane M1 and backplate BP pointing towardsthe sound port.

Schematic top views onto some selected layers of this embodiment areshown in FIG. 16. FIG. 16a shows the first membrane M1. It has just oneventing holes VH1 in the center of the membrane. This venting hole VHcauses an undercut in the underlying remaining portion of thesacrificial layer, a coupling, between the membranes M1-2 and thebackplate BP after the VHF etching process resulting in a gap in thecentral coupling portion CCP that is larger than the according vent holeVH in the membrane as shown in FIG. 16 b.

The backplate, as shown in FIG. 16c also has a small central ventinghole VH in the separated central area SCA. The separated central areaSCA has no connection to the rest of the backplate BP and can moveindependently from the rest of the backplate BP. Another remainingportion of the sacrificial layer, which is shown in FIG. 16d , couplesthe separated central area SCA to the second membrane M2. The secondmembrane M2 has, additional to the central venting hole VH piercing thecomplete microphone, a plurality of venting holes VH arranged over thearea outside of the central area CA, as can be seen in FIG. 16 e.

In FIG. 14 a schematic cross section of another embodiment of the MEMSmicrophone is shown. It differs from the embodiment shown in FIG. 13 byemploying bulges BU in the second membrane M2 and the backplate BP thatwork as respective stoppers reducing the maximum possible membranedisplacement. These bulges BU are manufactured in the layers by firstlyproducing depressions in the insulation layer IL2-3 directly below thelayer having the desired bulges BU by means of lithography and an RIEetching method. In the next step the partial layers of the functionallayer are deposited as usual. As the partial layers are close fitting tothe topography of the depressions of the insulation layer below, bulgesBU are formed this way.

FIG. 15 shows a schematic cross section of a further embodiment of theMEMS microphone. In this embodiment all the small vent holes VH1, withthe exception of the central vent hole VH, are omitted in the secondmembrane M2 compared to the embodiment shown in FIG. 13. Therefore, thespace between the first and second membranes M1, M2 is now sealed andallows to be evacuated or set under negative pressure. As a consequence,the noise measured by the backplate BP is reduced even further. Tomechanically withstand an applied or produced vacuum, it is necessary tostabilize the membranes M1, M2. Else the membranes M1, M2 couldcollapse. To prevent collapsing, coupling portions are introduced alongthe membranes M1, M2, which stabilize the membranes M1, M2. The couplingportions support the membranes M1, M2 by coupling them to the stiffbackplate BP. As in the central coupling portion CCP, the couplingportions are remaining portion of the sacrificial layers. Alternatively,it is also possible to implement bulges BU, similar to FIG. 14, tostabilize the membranes M1, M2 and keep them spaced apart from thebackplate BP.

It has to be noticed that the invention is not limited to the layoutsdescribed before and that further layouts can be retrieved bycombination of features taken from different figures and embodiments.

What is claimed is:
 1. A MEMS microphone comprising: a first membrane; abackplate with a separated central area comprising: a first backplateelectrode on a lower portion of the backplate; a second backplateelectrode on a upper portion of the backplate; and a backplateinsulation layer galvanically isolating the first and second backplateelectrodes; a second membrane; and a coupling central portion, whereinthe first membrane couples mechanically to the separated central area ofthe backplate in an electrically isolating manner and the separatedcentral area of the backplate couples to the second membrane in anelectrically isolating manner, and wherein the first membrane and/or thesecond membrane comprise(s) a single vent hole with a diameter smallerthan 0.5 μm.
 2. The microphone according to claim 1, wherein the venthole goes through all layers in the coupling central area.
 3. Themicrophone according to claim 1, wherein the second membrane has bulgesconfigured to constrain a displacement of the second membrane relativeto the backplate.
 4. The microphone according to claim 1, wherein thebackplate has bulges configured to constrain a displacement of the firstmembrane relative to the backplate.
 5. A method for manufacturing a MEMSmicrophone, the method comprising: a) on a substrate the followinglayers are deposited one above another: a first insulation layer and oneor a plurality of partial layers for a first membrane; b) the partiallayers of the first membrane are patterned by lithographic etching; c)above the first membrane the following layers are deposited: a secondinsulation layer and a plurality of partial layers for a backplatecomprising two conductive partial layers as backplate electrodes thatare galvanically isolated from each other; d) the partial layers of thebackplate are patterned by lithographic etching, separating a centralarea in a center of the backplate, a horizontal radial extension ofwhich is large compared to a vertical thickness of the second insulationlayer; e) a third insulation layer and one or a plurality of partiallayers for a second membrane are deposited; f) the second membrane ispatterned by lithographic etching; g) a fourth insulation layer isdeposited above the second membrane; h) openings to the electricallyconductive partial layers of the first membrane, the two backplateelectrodes and the conductive partial layer of the second membrane areetched and electrical contact pads are deposited therein; i) aperforation is etched through the substrate from a bottom side of thesubstrate below an active region of the first membrane to expose thelayer stack formed in step a) below the active region of the firstmembrane; and j) the insulation layers are removed in the active regionby an isotropic wet etching creating an airgap between both membranesand the backplate, wherein the isotropic wet etching is controlled toprevent a portion of the insulation layers under and above the separatedcentral area from being etched away, thereby creating two electricallyisolating volumes coupling mechanically in the separated central area toboth membranes and forming a rigid mechanical joint and a centralcoupling portion.
 6. The method according to claim 5, wherein thepartial layers and the insulation layers are deposited by LPCVD.
 7. Themethod according to claim 5, wherein in method step a) or e) a SiO₂layer is deposited as an insulation layer, thereabove as a first partiallayer a silicon nitride layer, as a second partial layer a polysiliconlayer and as a third partial layer a further silicon nitride layer isdeposited to assemble the first or second membrane.
 8. The methodaccording to claim 5, wherein in method step c) a SiO₂ layer isdeposited as the second insulation layer, thereabove as a first partiallayer a polysilicon layer, as a second partial layer a silicon nitridelayer and as a third partial layer a further polysilicon layer isdeposited to form the backplate.
 9. A method for manufacturing a MEMSmicrophone, the method comprising: depositing a first insulation layerfor a first membrane on a substrate; depositing one or more partiallayers for the first membrane on the first insulation layer; patterningthe one or more partial layers of the first membrane by lithographicetching; depositing on the first membrane a second insulation layer;depositing on the second insulation layer a plurality of partial layersfor a backplate comprising two conductive partial layers as backplateelectrodes that are galvanically isolated from each other; patterningthe partial layers of the backplate by lithographic etching; separatinga central area in a center of the backplate, wherein a horizontal radialextension of the central area is large compared to a vertical thicknessof the second insulation layer; deposing a third insulation layer andone or more partial layers for a second membrane; patterning the secondmembrane by lithographic etching; depositing a fourth insulation layeron the second membrane; etching openings to the electrically conductivepartial layers of the first membrane, the two backplate electrodes andthe conductive partial layer of the second membrane; depositingelectrical contact pads in the openings; etching a perforation throughthe substrate from a bottom side of the substrate below an active regionof the first membrane to expose a layer stack formed by depositing thefirst insulation layer and depositing the one or more partial layersbelow the active region of the first membrane; and removing theinsulation layers in the active region by an isotropic wet etchingthereby creating an airgap between both membranes and the backplate,wherein the isotropic wet etching is controlled to prevent a portion ofthe insulation layers under and above the separated central area frombeing etched away, thereby creating two electrically isolating volumescoupling mechanically in the separated central area to both membranesand forming a rigid mechanical joint and a central coupling portion. 10.The method according to claim 9, wherein the partial layers and theinsulation layers are deposited by LPCVD.
 11. The method according toclaim 9, wherein the second insulation layer is a SiO₂ layer, andwherein the plurality of partial layers for the backplate comprises afirst partial layer being a polysilicon layer, a second partial layerbeing a silicon nitride layer and a third partial layer being a furtherpolysilicon layer.
 12. The method according to claim 9, wherein thefirst insulation layer is a SiO₂ layer, and wherein the one or morepartial layers for the first membrane comprise a first partial layerbeing silicon nitride layer, a second partial layer being a polysiliconlayer and a third partial layer being a further silicon nitride layer.13. The method according to claim 9, wherein the third insulation layeris a SiO₂ layer, and wherein the one or more partial layers for thesecond membrane comprise a first partial layer being silicon nitridelayer, a second partial layer being a polysilicon layer and a thirdpartial layer being a further silicon nitride layer.
 14. A MEMSmicrophone comprising: a first membrane; a backplate with a separatedcentral area comprising: a first backplate electrode on a lower portionof the backplate; a second backplate electrode on a upper portion of thebackplate; and a backplate insulation layer galvanically isolating thefirst and second backplate electrodes; a second membrane; and a couplingcentral portion, wherein the first membrane couples mechanically to theseparated central area of the backplate in an electrically isolatingmanner and the separated central area of the backplate couples to thesecond membrane in an electrically isolating manner, and wherein thefirst membrane and the second membrane are mutually coupled mechanicallyjust in a center area of the membranes.
 15. The microphone according toclaim 14, further comprising at least one vent hole going through alllayers in the coupling central area.
 16. The microphone according toclaim 14, wherein the second membrane has bulges configured to constraina displacement of the second membrane relative to the backplate.
 17. Themicrophone according to claim 14, wherein the backplate has bulgesconfigured to constrain a displacement of the first membrane relative tothe backplate.